Online Drought Indicators and Indices

Standardized Precipitation Index (SPI)


Index name: Standardized Precipitation Index (SPI).

Ease of use: Green.

Origins: The result of research and work done in 1992 at Colorado State University, United States, by McKee et al. The outcome of their work was first presented at the 8th Conference on Applied Climatology, held in January 1993. The basis of the index is that it builds upon the relationships of drought to frequency, duration and timescales.

In 2009, WMO recommended SPI as the main meteorological drought index that countries should use to monitor and follow drought conditions (Hayes, 2011). By identifying SPI as an index for broad use, WMO provided direction for countries trying to establish a level of drought early warning.

Characteristics: Uses historical precipitation records for any location to develop a probability of precipitation that can be computed at any number of timescales, from 1 month to 48 months or longer. As with other climatic indicators, the time series of data used to calculate SPI does not need to be of a specific length. Guttman (1998, 1999) noted that if additional data are present in a long time series, the results of the probability distribution will be more robust because more samples of extreme wet and extreme dry events are included. SPI can be calculated on as little as 20 years’ worth of data, but ideally the time series should have a minimum of 30 years of data, even when missing data are accounted for.

SPI has an intensity scale in which both positive and negative values are calculated, which correlate directly to wet and dry events. For drought, there is great interest in the ‘tails’ of the precipitation distribution, and especially in the extreme dry events, which are the events considered to be rare based upon the climate of the region being investigated.

Drought events are indicated when the results of SPI, for whichever timescale is being investigated, become continuously negative and reach a value of -1. The drought event is considered to be ongoing until SPI reaches a value of 0. McKee et al. (1993) stated that drought begins at an SPI of -1 or less, but there is no standard in place, as some researchers will choose a threshold that is less than 0, but not quite -1, while others will initially classify drought at values less than -1.

Owing to the utility and flexibility of SPI, it can be calculated with data missing from the period of record for a location. Ideally, the time series should be as complete as possible, but SPI calculations will provide a ‘null’ value if there are insufficient data to calculate a value, and SPI will begin calculating output again as data become available. SPI is typically calculated for timescales of up to 24 months, and the flexibility of the index allows for multiple applications addressing events that affect agriculture, water resources and other sectors.

Input parameters: Precipitation. Most users apply SPI using monthly datasets, but the computer programs have the flexibility to produce results when using daily and weekly values. The methodology of SPI does not change based upon using daily, weekly or monthly data.

Applications: The ability of SPI to be calculated at various timescales allows for multiple applications. Depending on the drought impact in question, SPI values for 3 months or less might be useful for basic drought monitoring, values for 6 months or less for monitoring agricultural impacts and values for 12 months or longer for hydrological impacts. SPI can also be calculated on gridded precipitation datasets, which allows for a wider scope of users than those just working with station-based data.

Strengths: Using precipitation data only is the greatest strength of SPI, as it makes it very easy to use and calculate. SPI is applicable in all climate regimes, and SPI values for very different climates can be compared. The ability of SPI to be computed for short periods of record that contain missing data is also valuable for those regions that may be data-poor or lacking long-term, cohesive datasets. The program used to calculate SPI is easy to use and readily available. NDMC provides a program for use on personal computers that has been distributed to more than 200 countries around the world. The ability to be calculated over multiple timescales also allows SPI to have a wide breadth of application. Many articles relating to SPI are available in the science literature, giving novice users a multitude of resources to rely on for assistance.

Weaknesses: With precipitation as the only input, SPI is deficient when accounting for the temperature component, which is important to the overall water balance and water use of a region. This drawback can make it more difficult to compare events of similar SPI values but different temperature scenarios. The flexibility of SPI to be calculated for short periods of record, or on data that contain many missing values, can also lead to misuse of the output, as the program will provide output for whatever input is provided. SPI assumes a prior distribution, which may not be appropriate in all environments, particularly when examining short-duration events or entry into, or exit out of, drought. There are many versions of SPI available, implemented within various computing software packages other than that found in the source code distributed by NDMC. It is important to check the integrity of these algorithms and the consistency of output with the published versions.

Resource: The SPI program can be run on Windows-based personal computers. Download at: National Drought Mitigation Center – SPI Program. Global SPI data at 3-, 6-and 12-moth scales is available at NCAR/UCAR Research Data Archive. Visualization of global SPI data at 3-, 6- and 12-month scales is provided by the Tokyo Climate Center (TCC) ClimatView tool. Additional resources are available at the Flood and Drought portal developed by GEF, UN Environment, IWA and DHI.

Guttman, N.B., 1998: Comparing the Palmer Drought Index and the Standardized Precipitation Index. Journal of the American Water Resources Association, 34: 113–121. DOI: 10.1111/j.1752-1688.1998.tb05964.x. (For more information on this paper, please contact the IDMP HelpDesk).

Guttman, N.B., 1999: Accepting the Standardized Precipitation Index: A Calculation Algorithm. Journal of the American Water Resources Association, 35: 311–322. DOI: 10.1111/j.1752-1688.1999.tb03592.x. (For more information on this paper, please contact the IDMP HelpDesk).

Hayes, M., M. Svoboda, N. Wall and M. Widhalm, 2011: The Lincoln Declaration on Drought Indices: Universal Meteorological Drought Index Recommended. Bulletin of the American Meteorological Society, 92(4): 485–488. DOI: 10.1175/2010BAMS3103.1.

McKee, T.B., N.J. Doesken and J. Kleist, 1993: The Relationship of Drought Frequency and Duration to Time Scales. Proceedings of the 8th Conference on Applied Climatology, 17–22 January 1993, Anaheim, CA. Boston, MA, American Meteorological Society.

World Meteorological Organization, 2012: Standardized Precipitation Index User Guide, (WMO-No. 1090), World Meteorological Organization, Geneva, Switzerland.

Wu, H., M.J. Hayes, D.A. Wilhite and M.D. Svoboda, 2005: The Effect of the Length of Record on the Standardized Precipitation Index Calculation. International Journal of Climatology, 25(4): 505–520. DOI: 10.1002/joc.1142.

Currently used by: Argentina, Austria, Belize, Bosnia and Herzegovina, Brazil, Bulgaria, Canada, Chile, Croatia, Cyprus, Dominican Republic, Germany, Greece, Hong Kong, Iran, Israel, Jamaica, Jordan, Kazakhstan, Libya, Lithuania, Macedonia, New Zealand, Pakistan, Peru, Slovenia, Spain, Sri Lanka, Switzerland, Tanzania, Thailand, Trinidad and Tobago, Turkey, Ukraine, USA.

  1. Berecha Teressa
    May 7, 2021 Reply

    i need spi softwere

    1. Katrin Ehlert
      May 11, 2021 Reply

      Dear Berecha Teressa

      Here you go:

      You can download the software and documentation by clicking on “Program and Documentation” on the right-hand side of the page.

      Best regards
      IDMP TSU

  2. Shani Khan
    February 8, 2021 Reply

    How to prepare the data sheet for calculating SPI?

    1. Henri
      March 28, 2021 Reply

      tus datos históricos tienes q ordenarlos en dos columnas (Fecha; Precipitacion (mm)) para que puedas trabajarlos. saludos

  3. Shani Khan
    February 8, 2021 Reply

    i want to calculate the SPI for 6 month time scale. I have 10 different rainfall stations of one region. can anyone tell me how to arrange the data for calculation of SPI. I have the data for same time e.g 1981 to 2020 for all stations.

    1. Katrin Ehlert
      April 8, 2021 Reply

      Dear Shani Khan

      The SPI program uses two input parameters – the date and precipitation amount. Please consult the documentation of the SPI tool for detailed information on how to format input files. The documentation is part of the software package that can be downloaded here:
      Regarding your analysis of various stations, you can generate the SPI for each station by preparing an input file for each station.

      Best regards
      IDMP TSU

      1. Angad
        May 30, 2021 Reply

        The link is not working any idea to get it

        1. Katrin Ehlert
          June 1, 2021 Reply

          Dear Angad
          The website is temporarily offline, but will go online soon again. The link is correct, please try again to see the status.
          Best regards
          IDMP TSU

  4. shafique Ur Rehman
    January 27, 2021 Reply

    Hi how to extract severity and duration after calculating SPI

    1. Robert Stefanski
      February 16, 2021 Reply

      Dear Shafique,

      Thank you for your question. The severity of the SPI is more negative number. Please note the SPI is standardized. Also, you will need to compare the SPI calculations with the historical occurrences and impacts.

      Best regards,

      IDMP TSU

  5. shafique Ur Rehman
    December 31, 2020 Reply

    If this comment is in page kindly send again

  6. shafique Ur Rehman
    December 31, 2020 Reply

    How to obtain Severity and Duration variables from calculated SPI. Anyone can help me

  7. shafique Ur Rehman
    December 31, 2020 Reply

    Dear all anyone tell me how to seprate duraton and severity variables from calculated SPI.
    I have successfullu calculate the SPI values from different stations but dont know how to identiify the sevrity and duration vaiables.

  8. shafique Ur Rehman
    November 15, 2020 Reply

    If I click on executable file then SPI generator file is seen and not understand about documentation how to carry input file and SPI are obtain

    1. Robert Stefanski
      February 16, 2021 Reply

      Dear Shafique,

      Thank you for your question. Please let us know if you are still having a problem using the SPI downloadable file with Windows 10. We will check with the US National Drought Mitigation Center who are the developers of the software to see there are any recurring issues. Here is the link to their website.

      Best regards,
      IDMP TSU

  9. shafique Ur Rehman
    November 15, 2020 Reply

    Sir how to install SPI software in pc. i am using window 10 in pc. If i download the document and programming file then no setup is seen in that folder. kindly help me about that. Thankuu

    1. shafique Ur Rehman
      November 15, 2020 Reply

      If I click on executable file then SPI generator file is seen and not understand about documentation how to carry input file and SPI are obtain

  10. sachini wijesundara
    November 3, 2020 Reply

    I’m going to compare drought events between two cities. There I’m using only 12 years of data(2007-2018).
    For one town spi values related to February is NA(for the whole period only February data). I don’t why. But the software returned the February data for the other town. Please help me in this regard.

    1. Robert Stefanski
      February 16, 2021 Reply

      Dear Sachini,

      Thank you for your question. This is a detailed question. We will email you directly if there is still a problem.

      Best regards,
      IDMP TSU

  11. Nurul
    September 29, 2020 Reply

    How to calculate SPI for each grid from gridded data in nc file?

    1. Robert Stefanski
      February 16, 2021 Reply

      Dear Nurul,

      Thank you for your question. Have taken a look at the SPI Guide?

      Best regards,
      IDMP TSU

  12. Abera Bekele
    December 28, 2019 Reply

    which models is more appropriate to forecasting drought a head of time of three month through using SPI? specially for the semi-arid areas of Oromia pastoral areas of Ethiopia, How will I get SPI soft were? could I use the rain fall data from local station to get SPI result of my interest area?

    1. Katrin Ehlert
      February 3, 2020 Reply

      Dear Abela Bekele

      Thank you for your comment.
      As for a model that will deliver the best SPI predictions 3 months ahead of time, this is dependent on which model works best for your local context. Your national Meteorlogical Service may have information on which model performs best in your region of Ethiopia. Once you have decided on a model, you can use the SPI generator application ( to run gridded projected precipitation values for your chosen time period. In this case, the program treats gridded data as station data.
      You can use the precipitation data from your local station to calculate the SPI. Please note that for a viable result, you will need good monthly precipitation data for at least the past 25-30 years. A longer data track is always better in order to get more reliable results.

      Best regards
      IDMP team

  13. Endale
    November 16, 2019 Reply

    please attach to me SPI software packages

    1. Katrin Ehlert
      December 6, 2019 Reply

      Dear Endale

      Thank you for your comment.

      For example, you can find the R-based SPI program here:

      Best regards
      IDMP team

  14. Shahla
    September 1, 2019 Reply

    How to take timescales in computing spi? Taking them in the cumulative probability or in precipitation data? Can you share me a theoretical step by step computation

    1. Robert Stefanski
      September 2, 2019 Reply

      Dear Shahla,

      Thank you for your email. If we understand your request correctly, it is recommended to use 30 years of precipitation data. You can use 10 years but then the analysis will not be robust.

      Please look at the references at the bottom of the SPI web page. Specifically, look at the Standardized Precipitation Index User Guide published by WMO.

      Best regards,
      IDMP Team

  15. desta sulamo
    July 25, 2019 Reply

    i am thankful for the realization of this simplified software

  16. Your name...
    June 15, 2019 Reply

    How to calculate spi for missing data and zero rainfall

    1. Katrin Ehlert
      June 17, 2019 Reply

      Dear Santoshchougale.27

      Thank you for your comment.
      The Standardized Precipitation Index (SPI) User Guide provides an explanation of the data needed in order to calculate the SPI. You can find it here:

      This extract of the above-mentioned publication lists the key points on how the SPI works:
      • Precipitation is normalized using a probability distribution function so that values of SPI are
      actually seen as standard deviations from the median.
      • A normalized distribution allows for estimation of both dry and wet periods.
      • Accumulated values can be used to analyse drought severity (magnitude).
      • At least 30 years of continuous monthly precipitation data are needed but longer-term
      records would be preferable.
      • SPI timescale intervals shorter than 1 month and longer than 24 months may be unreliable.
      • It is spatially invariant in its interpretation.
      • Its probability-based nature (probability of observed precipitation transformed into an index)
      makes it well suited to risk management and triggers for decision-making.

      Kind regards,
      IDMP Team

  17. Nasrin Salehnia
    April 5, 2019 Reply

    Hi Jaldesa
    About drought forecasting, First of all, you should obtain precipitation values of the future period via outputs of CMIP5 models, or weather generators, or etc., then you can put them in the MDM ( tool or other tools for calculating rain-based drought indices.
    When you extract your specific data from CMIP5 models under RCP scenarios, you should use statistical downscaling methods ( or dynamical downscaling methods for finer resolutions then apply them in your drought monitoring tool for the future period. Also, be careful for implementing calibration and validation steps
    If you have further question let me know (

  18. Sohrab Kolsoumi
    April 2, 2019 Reply

    Hi Robert Stefanski and Hassan
    I checked the DrainC tool and compare it. Also, I focused on the formula I think the result of Drainc is not true. But the Drought Mitigation Center software for SPI is similar to DMAP and it is true.

  19. Sohrab Kolsoumi
    April 2, 2019 Reply

    Hi Jaldesa Doyo
    If you want to predict drought so you need to future climate data. You can use weather generators such as KNN-WG.
    The best prediction for drought prediction is GCM models. because GCM models have been run by dynamic models based on RCP scenarios. RCP scenarios consider climate change.
    You can use SD-GCM tool for downscaling GCM data. If you want to use coarse scale so you can use DMAP V1.1.
    Good Luck

  20. Sohrab Kolsoumi
    April 2, 2019 Reply

    Hi Eremugo Isaac
    You can use these two indexes:
    SWSI (Surface Water Supply Index), Garen, 1993
    SDI (Streamflow Drought Index), Nalbantis and Tsakiris, 2009
    You can use this tool for calculating:
    I can not add here an excel file but if you want I can send to you an excel file for calculating SWSI.

  21. Jaldesa Doyo
    February 25, 2019 Reply

    I am to get this vital information from you, My question is that , how we can calculate future drought / drought forecast for drought monitoring by SPI and RDI?

    1. Robert Stefanski
      August 13, 2019 Reply

      Dear Jaldesa,

      SPI and RDI are indices and therefore you can use weather / climate forecasts to calculate the indices in the future.

      Another example is provided by the USA. Please see the following examples of a drought monitoring map linked to the drought forecast.
      US Drought Monitor –

      Drought Forecast –

      Best regards,
      IDMP Team

  22. Eremugo Isaac
    February 6, 2019 Reply

    Dear Team,

    Kindly share with me a guide for Stream-flow Drought Index

    Best Regards

    1. Robert Stefanski
      August 12, 2019 Reply

      Dear Eremugo Issac,

      Here is the refernce:
      Nalbantis, I. and G. Tsakiris, 2009: Assessment of hydrological drought revisited. Water Resources Management, 23(5): 881–897. DOI 10.1007/s11269-008-9305-1.

      Best regards,

  23. Atilaw Amare
    January 31, 2019 Reply

    It is good teaching about SPI

  24. Hassan Sheidaee
    December 10, 2018 Reply

    I used 3 tools for calculating SPI:
    1- DrainC
    3-Drought Mitigation Center software for SPI
    But I faced two difference between the calculated values of SPI in items1 Veras Item2,3
    Which of them is true?
    All of them has a research paper in peer-reviewed journals.


    1. Robert Stefanski
      January 9, 2019 Reply

      Dear Hassan,

      Thank you for your email. It is always go to look at different formulas. Are there significant differences in the three methods? This may be an interesting journal article. However, it not a question of which one of them is true but which one best represents drought in your country and region.

      We do know that the Drought Mitigation Center software for SPI is widely used.

      Best regards,


  25. Befirdu Zeleke
    December 7, 2018 Reply

    I need to have SPI software temple to download please if you have send to me. And Mann-Kendal software temple.
    Best regards,

    1. Robert Stefanski
      January 9, 2019 Reply

      Dear Befirdu Zeleke,

      Thank you for your email. Each index has a section where you can find the software (if it exists).

      The SPI software can be found at:

      We are not familiar with the Mann-Kendal software. It is not listed in the IDMP Handbook on Drought Indicators and Indices.

      Could you please find more us information on this index??

      Best regards,


  26. Nwet Nwet
    September 25, 2018 Reply

    Please,Can I have you?How do the download for spi software?

    1. Robert Stefanski
      January 9, 2019 Reply

      Dear Nwet Nwet,

      Thank you for your email. Each index has a section where you can find the software (if it exists).

      The SPI software can be found at:

      Best regards,


  27. Kris Correa Marrou
    August 2, 2018 Reply


    Can you explain me or give me reference about -0.8 SPI value as a threshold of moderate drought? It will serve to research in Peru.

    Thanks a lot.

    1. Robert Stefanski
      August 8, 2018 Reply

      Dear Kris Correa Marrou,

      The threshold on any drought index including the SPI have to be referenced to historical events and impacts.
      Please look at the following publications:
      SPI guide at;
      the Handbook on Drought Indices at
      and the National Drought Management Policy Guidelines at
      Best regards,

      IDMP Team

  28. Kris Correa Marrou
    July 30, 2018 Reply

    Can you explain me or give me a reference about -0.8 SPI value as a threshold of moderate drought? It will serve to research in Peru.

    Thanks a lot.

  29. Galgalo Wako
    July 9, 2018 Reply

    my question regarding this SPI 1,is what are the softwares we can calculate SPI on 2, for how many year that we have to calculate that the result have to be accurate 3, what are the accurate classes for the drought in pastoralist areas e.g moderate ,severy and others or standard numbers are

    1. Robert Stefanski
      August 8, 2018 Reply

      Dear Galgalo Wako,

      Here are the answers to your questions.

      1) Please find the SPI Guide at

      2) According the above guide, the minimum dataset is 30 years of rainfall data. Here the exact text from page 7 of the SPI guide:

      “… recommendation of having around 50–60 years of data available. Unless one has 80–100 years of data, the sample size is too small and the statistical confidence of the probability estimates on the tails (both wet and dry extremes) becomes weak beyond 24 months. In addition, having only the minimum 30 years of data (or less) shortens the sample size and weakens the confidence. Technically, one could run the SPI on less than 30 years of data bearing in mind, however, the statistical limitations and weaker confidence pointed out above.”

      3) The SPI can be used for many applications but you will need to know the historical droughts and impacts. We will need to obtain the previous droughts impacting pastoralists then you could determine which classes or thresholds to use for your country or region.

      IDMP Team

    March 17, 2018 Reply

    how you culcul the spi index with the logiciel arcgis ?

    1. Robert Stefanski
      March 22, 2018 Reply

      Dear Meka,

      Thank you for your comment. Please look at the Standardized Precipitation Index
      User Guide at Please look at page 13-15 which discusses mapping capabilities.

      Best regards,


  31. temesgen fikadie
    June 30, 2017 Reply

    softwares about standardized preciptation index and detail statsitcal calculation and working procedure about standardized streamflow indx(ssfi), streamflow drought index(sdi), drought reconenesance index(dri) ,percent normal precipitation, deciles

    1. Robert Stefanski
      July 13, 2017 Reply

      Please be sure to read the description of all of the indices. Much of information that you have requested is on the web page of each index.
      For the SPI:
      DRI software is available at

    2. Thabang Mashiloane
      October 28, 2020 Reply

      Good day sir I would like to know how are SPI values interpreted once calculated and classified.

      1. Robert Stefanski
        February 16, 2021 Reply

        Dear Thabang,

        Please take a look at Tables 1 and 2 in the SPI Guide. The classification scheme can also be adjusted to your actual needs. This is where you will need to know the historical impacts on drought in your country or location.

        Best regards,
        IDMP TSU

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